INSTALL.md: update instructions for ArchLinux

[ArchangeGabriel]

The main package has been proposed in the [community] repository for some time already.

[welcome]

💖 Thanks for opening this pull request! 💖

Please make sure you read our Contributing Guide and abide by our Code of Conduct.

A few things to keep in mind:

• If you need help writing tests, take a look at the existing ones for inspiration. If you don't know where to start, let us know and we'll walk you through it.
• All new features should be documented. It helps to write the comments below for your functions describing the it, all arguments, and return types before writing the code. This will help you think about your code design and results in better code.
• We are grateful that you put in the effort to do this! 🎉

[PaulWessel]

Thanks!

[welcome]

🎉🎉🎉 Congrats on merging your first pull request and welcome to the team! 🎉🎉🎉

We hope that this was a good experience for you. Let us know if there is any way that the contributing process could be improved.